Viewports broken on RX 580
Started having the same problem just after a Radeon update[I think 20.11.1]  (AMD RX580 8gb),
GPU viewport rendering is broken(was using portable Blender folder, tried installing updated Blender and still no fix).
After some googling found some people were having similar problems and decided to leave it for a couple of weeks in case of a fix-patch.
No update patch yet but I tried switching to &quot;High Quality Normals&quot; and it totally worked for me!
TEMPORARY FIX: Render Properties &gt; Performance &gt; High Quality
Hope this can help others too.